Thermowells are recommended whenever a temperature element is to be inserted into a process where corrosion, pressure, abrasion, or shear forces may threaten the life of the element. In addition thermowells allow for a defective instrument to be removed without shutting down or draining the process. They fit into 3 broad categories: Screwed, Flanged, or Weld-in, and can be designed to accept Thermocouples, RTD’s Temperature Gauges or filled systems. Thermowells can be machined from solid barstock, or fabricated from pipe.
Fabricated thermowells offer a lower cost alternative where high pressure is not a design consideration. Standard thread forms are BSP and NPT (others available to order), and flange size and rating can be supplied to suit any ones requirement. Material certification, and full NDT requirements such as Dye penetrant, Hydrostatic Pressure Testing, Radiography, and Post Weld Heat Treatment can all be provided. Wells can also be produced to satisfy the requirements of NACE MR 01 75. Stress and Frequency calculations can be performed to ensure that the thermowell design is suitable for the required process conditions.
Thermowell Material Specification
Low cost materials with little corrosion resistance. Used in low temperature and stress applications where the measured medium protects it from corrosion, oils, petroleum, tars etc.
Maximum temperature: 530°C.
Chrome/Moly Steels (F11, F22)
High strength steels used in pressure vessels and industrial boiler plant. Resistant to chloroform, cleaning agents, food products and carbon disulphide.
Maximum temperature: 600°C.
304 Stainless Steel
Low cost corrosion resistant material, used extensively used in food, beverage and chemical processing where good corrosion resistance is required. A low carbon grade, 304L is available which can be welded without impairing it’s corrosion resistance.
Maximum temperature: 900°C.
321 Stainless Steel
Similar properties to 304SS above, except that this grade is titanium stabilised to prevent intergranular corrosion when welded.
316 Stainless Steel
Best corrosion resistance of the austenitic stainless steels due to the addition of molybdenum, widely used in chemical processing, offers useful resistance to H S. As with 304, a low carbon grade, 316L is available for welded applications.
Maximum temperature: 900o C.
310 Stainless Steel
Heat resistant material, which can be used up to 1150°C with useful resistance in sulphur bearing atmospheres. Corrosion resistance is slightly better than 304SS, but not as good as 316SS. Can be welded with caution.
446 Stainless Steel
Ferritic stainless steel with excellent resistance to sulphurous atmospheres at high temperature, however due to its low strength at high temperature, thermowells made from this material should be mounted vertically. Used in heat treatment processes, iron and steel furnaces, gas production plant and it has some useful resistance to molten lead. Good corrosion resistance to nitric acid, sulphuric acid and most alkalis gives it some limited use in chemical plant.
Maximum temperature: 1150°C.
Duplex Stainless Steel
These grades combine high strength with excellent corrosion resistance, especially to chloride stress corrosion cracking, however a tendency to brittleness limits their use to approx. 300 to 315°C maximum, sub- zero use is also restricted to approx 50°C because of brittleness due to the ferrite content. Main uses include offshore piping, chemical tanks, flue gas scrubbers and chimneys.
Super Duplex Stainless Steel
Similar qualities and limitations to duplex grades above. These grades are widely used to handle seawater and other brackish waters, marine pumps, oil and gas production and desalination plants are typical applications.
A very widely used nickel-chromium- ion alloy with excellent high temperature strength and oxidation resistance, however it is very vulnerable to attack in sulphurous atmospheres above 500°C. Good resistance to chloride-ion stress corrosion cracking and nitriding environments. Use extensively in chemical industries for its strength and corrosion resistance. Easily welded, can normally be used without post weld heat treatment.
Maximum temperature: 1212°C.
A nickel-chromium alloy with excellent resistance to pitting and crevice corrosion, unaffected by radiation embrittlement, widely used in aerospace applications and in marine environments. Good weldability, can be used in the as-welded condition. Maximum temperature: 1093°C.
Superior to alloy 600 in sulphur, cyanide salts and neutral salts. Extensively used in steam/hydrocarbon reforming plants for pigtail piping, manifolds and waste heat boilers and in the internal components of secondary reformers. Widely used in heat treatment equipment and as a heater sheath material.
Maximum temperature: 1093°C.
A nickel- iron-chromium alloy with exceptional resistance to many corrosive environments, it is superior to almost all metallic materials for handling sulphuric acid and shows excellent resistance to phosphoric acid. It is widely used in chemical processing, oil and gas recovery, acid production, pickling operations, nuclear fuel reprocessing and handling of radioactive wastes.
Maximum temperature: 540°C.
An iron-chromium-aluminium alloy produced by mechanical alloying, it features great strength at high temperatures with excellent resistance to oxidation, carburisation, and hot corrosion. These properties make it especially suitable for gas turbine combustion chambers and other aggressive environments containing sulphur and chloride salts.
Maximum temperature: 1350°C.
Nickel-copper alloy with very good corrosion resistance, commonly used to handle sea water, hydrofluoric acid, sulphuric acid, hydrochloric acid and most alkalis. Typical applications include marine fixtures, chemical processing equipment, gasoline and water tanks, process vessels and piping and boiler feedwater heaters.
Maximum temperature: 538°C.
Nickel-molybdenum-chromium alloy with excellent corrosion resistance especially in chlorinated environments. Widely used in chemical plant where it tolerates ferric and cupric chlorides, solvents, chlorine, formic acid, acetic acid, brine, wet chlorine gas and hypochlorite. Can be easily welded and maintains its properties in the as-welded condition.
Maximum temperature: 1093°C.
A development of the well- established B2 alloy with improved thermal stability, fabricability and stress corrosion cracking resistance. It is the alloy of choice for handling hydrochloric acid in all concentrations and temperatures; it also withstands hydrogen chloride, sulphuric, acetic, hydrofluoric and phosphoric acids.
Maximum temperature: 538°C, up to 816°C in reducing or vacuum environment.
A nickel-chromium-molybdenum-tungsten alloy with outstanding resistance to pitting, crevice corrosion and stress corrosion cracking. It shows exceptional resistance to a wide range of chemical process environments, such as ferric and cupric chlorides, chlorine, hot contaminated solutions, formic and acetic acids and seawater or brine solutions. The material has superior weldability, and retains its properties in the aswelded state.
A high temperature alloy with excellent resistance to oxidising, reducing and neutral atmospheric conditions. Is widely used in aircraft jet engine components. Very good high temperature strength makes it ideal for furnace applications. Resistant to stress corrosion cracking in petrochemical applications.
Maximum Temperature: 1204°C.
Haynes alloy 214
Excellent high temperature material, with the highest resistance to oxidation and carburisation of almost any alloy, it is recommended for temperatures of 950°C and above, it shows useful resistance up to 1315°C but its strength is severely reduced. Applications include ceramic firing furnace parts, automotive catalytic converter internals and in the industrial heating industry furnace flame hoods and rotary calciners. Has good resistance to chlorine contaminated environments, which allows its use in hospital waste incinerators.
Maximum temperature: 1204°C.
Haynes alloy 230
Excellent high temperature strength, oxidation resistance and long term thermal stability. Used in aerospace, chemical processing and high temperature heating applications. Recommended for use in nitriding environments.
Maximum temperature: 1149°C.
Haynes alloy 556
A multipurpose alloy which offers good resistance to sulphidising, carburising and chlorine bearing atmospheres. Common applications include waste incinerators, petroleum processes where sulphur is present, chloride salt baths, exhaust gas probes, the alloy is one of very few that can survive in molten zinc, making itideal for galvanising processes.
Maximum temperature: 1093°C.
Haynes alloy HR160
A high temperature alloy with outstanding resistance to high temperature corrosion, it has excellent resistance to sulphidation, and chloride attack in both reducing and oxidising atmospheres. Its resistance to attack by the products of combustion of low- grade fuels makes it particularly useful in municipal, industrial, hazardous and nuclear waste incinerators.
Maximum temperature: 1204°C.
A lightweight material with good strength in the 150 to 470°C range. Excellent resistance to oxidising acids such as nitric or chromic, it is also resistant to inorganic chloride solutions, chlorinated organic compounds and moist chlorine gas. Its good resistance to seawater and salt spray allows it to be used in offshore installations. Can be welded with special precautions to protect from atmospheric contamination.
Refractory metal that is very ductile. Use only in inert atmospheres or very good vacuums. <10(-3) torr. Hydrogen and nitrogen will react with tantalum above 750°F (400°C) resulting in nitride and hydride formation that will affect life. Tantalum is almost completely immune to chemical attack at temperatures below 150°C, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide. Alkalis attack it only slowly. At high temperatures, tantalum becomes much more reactive. It is normally used by cladding a thin layer onto a less expensive material, such as steel or stainless steel.
Tantalum’s corrosion resistance is similar to that of glass, making it the material of choice in critical chemical and pharmaceutical processes requiring maximum corrosion resistance and minimum contamination. Typical applications are: Chemical production (HCl, H2 SO4, insecticides, pharmaceuticals, fine chemicals, explosives, plastics, dyestuffs, condensation of phosgene derivatives, synthetic fibres, chromic acid plating solutions and operations involving chlorine, bromine, iodine and their compounds, high purity product manufacturing (cosmetics, soaps, perfumes) and separation of high boiling organic acids.
THERMOWELL METAL PROPERTIES
STANDARD THERMOWELL SPECIFICATIONS
- All Fabricated thermowells to be used with RTDs are supplied with drilled solid tip
- Weld Procedure and qualifications on request
- All well can be supplied with lagging extensions
- If hex bar is unavailable for material specified, 2 spanner flats milled
- Plug and Chain available for all constructions
- Maximum machined thermowell length: 1200mm